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4.5CH4 Electricity and Magnetism

Electromagnetic Effects — Cambridge O Level Physics 5054 Revision Notes & Topical Questions

10 min read 14 parts Lesson 5 of 6

📚 Cambridge O Level Physics 5054 Hub / Topical Revision
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Part 1 of 12Magnetic effect of a current

  • Oersted's discovery
    • a current in a wire produces a magnetic field around it.
  • Straight wire
    • the field lines form concentric circles around the wire.
    • The field is stronger close to the wire and weaker with distance, so the circles are closer together near the wire.
  • Right-hand grip rule
    • point the thumb along the conventional current and the curled fingers show the field direction.
  • Reversing the current
    • it reverses the direction of the field.
  • Flat coil and solenoid
    • a solenoid is a long coil whose field looks like that of a bar magnet.
    • Inside the solenoid the field is strong and uniform, and the end where field lines emerge is the north pole.
  • Strengthening an electromagnet
    • increase the current, add more turns of wire, or put a soft iron core inside the coil.
    • Reversing the current reverses the poles, and switching off removes the magnetism.
  • Uses of electromagnets
    • scrapyard lifting, electric bells, relays, circuit breakers and loudspeakers.

Part 2 of 12Force on a current-carrying conductor

  • Motor effect
    • a wire carrying a current in a magnetic field experiences a force.
    • The magnetic field of the wire interacts with the field of the magnet.
  • Fleming's left-hand rule
    • hold the thumb, first finger and second finger at right angles.
FingerRepresents
First fingerField direction (north to south)
Second fingerCurrent direction (conventional)
ThumbForce (motion)
  • Reversing
    • reversing either the current or the field reverses the force, and reversing both leaves it unchanged.
  • Size of the force
    • it increases with a stronger magnet, larger current and a longer wire in the field.
    • There is no force if the wire is parallel to the field.
  • Demonstration
    • a wire swings between the poles of a horseshoe magnet when a current is switched on.
  • Beams of charged particles
    • a moving charge in a magnetic field also feels a force, which bends the path of electrons in a cathode-ray tube.

Part 3 of 12The d.c. motor

  • Motor
    • a device that converts electrical energy into kinetic energy using the force on a current in a field.
  • Parts
    • a rectangular coil, permanent magnets, a split-ring commutator and carbon brushes.
  • Operation
    • follow the cycle of events.
  1. Current flows through the coil from the brushes via the commutator.
  2. The two sides of the coil carry current in opposite directions in the field, so one side is pushed up and the other down.
  3. The forces make the coil rotate.
  4. At the vertical position the commutator reverses the current in the coil.
  5. The forces keep turning the coil in the same direction.
  • Increasing the speed or turning force
    • use a stronger magnet, more turns, a larger current or a soft iron core.
  • Reversing the direction
    • reverse the current or reverse the magnetic field.
  • Loudspeaker
    • a coil attached to a cone sits in a magnetic field, and the a.c. signal moves the coil and cone back and forth to produce sound.

Part 4 of 12Electromagnetic induction

  • Induction
    • an e.m.f. is induced in a conductor when it cuts magnetic field lines, or when the magnetic field through a coil changes.
  • Ways to induce an e.m.f.
    • move a magnet in or out of a coil, move a wire across a field, or change the current in a nearby coil.
    • No e.m.f. is induced when there is no relative movement.
  • Factors that increase the induced e.m.f.
    • move faster, use a stronger magnet, or use more turns in the coil.
  • Direction
    • reversing the movement or the polarity of the magnet reverses the direction of the induced e.m.f.
  • Lenz's law
    • the induced current flows in a direction so that its magnetic effect opposes the change that produced it.
    • This is an example of conservation of energy, because work must be done to push the magnet in.
  • Demonstration
    • connect a sensitive meter to a coil, push a magnet in and the needle deflects one way, pull it out and it deflects the other way.

Part 5 of 12The a.c. generator

  • Generator
    • a device that converts kinetic energy into electrical energy by rotating a coil in a magnetic field.
  • Parts
    • a coil, magnets, slip rings and brushes.
    • Slip rings keep the coil connected to the external circuit while it rotates, so the output is alternating.
  • Output
    • the e.m.f. is zero when the coil sides move parallel to the field and maximum when they cut it at right angles.
  • Graph
    • the output is a sine-shaped wave of alternating voltage with a period equal to one rotation.
  • Increasing the output
    • spin the coil faster, use stronger magnets or more turns, or use a larger coil area.
    • Faster rotation increases both the size and the frequency of the output.
  • Power stations
    • turbines turn large coils of generators, and the a.c. produced is sent to the grid.

Part 6 of 12Transformers

  • Transformer
    • a device that changes the size of an alternating voltage, using two coils wound on a soft iron core.
  • Operation
    • an alternating current in the primary coil produces a changing magnetic field in the core.
    • The changing field induces an alternating e.m.f. in the secondary coil.
  • Transformer equation
    • Vp ÷ Vs = Np ÷ Ns.
      What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
    • Step-up transformers have more turns on the secondary coil, and step-down transformers have fewer.
  • Ideal transformer
    • it is 100% efficient, so power in equals power out: IpVp = IsVs.
    • When the voltage is stepped up the current is stepped down.
  • Only works with a.c.
    • a steady d.c. gives no changing field, so no voltage is induced.
  • Energy losses
    • heating in the coils, eddy currents in the core (reduced by laminating it) and magnetic leakage.
      • (reduced by thick wire)

Part 7 of 12Power transmission

  • Why transmit at high voltage
    • for a given power, a higher voltage means a lower current (I = P ÷ V).
    • The heating loss in the cables is I2R, so a small current greatly reduces wasted energy.
  • Grid system
    • a step-up transformer at the power station raises the voltage to over 100 kV, and step-down transformers near homes lower it to 230 V.
  • Advantages of a.c.
    • it can be stepped up and down easily with transformers.
  • Safety
    • overhead lines are held on tall pylons and insulators to keep the high voltage away from people.

Part 8 of 12Demonstrating induction and Lenz's law

  • Experiment for induction
    • use a coil and a centre-zero galvanometer.
  1. Connect a coil of many turns to a sensitive centre-zero meter.
  2. Push the north pole of a bar magnet into the coil and watch the needle flick.
  3. Hold the magnet still inside the coil and the needle returns to zero.
  4. Pull the magnet out and the needle flicks the opposite way.
  5. Repeat faster, or with more turns, and the flick is bigger.
  • What changes the e.m.f.
    • the rate of change of the field, or the rate at which field lines are cut, and the number of turns in the coil.
  • Demonstrating Lenz's law
    • drop a magnet through a copper or aluminium tube, which falls slowly.
    • The induced currents in the tube produce a field that opposes the magnet's motion,
      • so the magnet falls more slowly than it would through a plastic tube.
  • Direction rule
    • pushing a north pole into a coil induces a north pole at that end of the coil, which repels the magnet.

Part 9 of 12The a.c. generator in detail

  • Rotating coil form
    • the coil turns between fixed magnets and is connected to the outside circuit by slip rings and brushes.
  • Rotating magnet form
    • a magnet turns inside fixed coils, so no slip rings are needed because the coils stay still.
  • E.m.f. against time
    • the output is a repeating wave.
Position of the coilE.m.f.
Coil vertical, sides moving along the fieldZero
Coil horizontal, sides cutting field lines at right anglesMaximum (peak or trough)
Coil vertical again after half a turnZero, with the opposite sign to follow
  • Interpreting the graph
    • the time for one full wave is one rotation, so faster rotation shortens the period and raises both the frequency and the peak e.m.f.

Part 10 of 12Relays and loudspeakers

  • Relay
    • a small current in a coil magnetises an iron core, which pulls an armature that closes contacts in a separate circuit.
    • It lets a small, safe low-voltage circuit switch a large current or a mains circuit, as in car starter motors and heating controls.
  • Loudspeaker
    • a coil attached to a paper cone sits in the field of a permanent magnet.
  1. The varying current from an amplifier flows through the coil.
  2. The force on the coil changes direction with the current.
  3. The coil and cone move back and forth with the same frequency as the signal.
  4. The cone pushes the air to make a sound wave.

Part 11 of 12Forces between parallel currents

  • Field around a wire
    • each wire has circular magnetic field lines around it.
  • Currents in the same direction
    • the fields between the wires partly cancel and the wires are pulled toward each other.
  • Currents in opposite directions
    • the fields between the wires reinforce and the wires are pushed apart.
  • Link to the motor effect
    • each wire is a current in the field of the other wire, so each feels a force given by Fleming's left-hand rule.
    • The forces on the two wires are equal in size and opposite in direction, as Newton's third law requires.

Part 12 of 12Worked examples

  1. A transformer has 400 turns on the primary coil and 100 turns on the secondary coil. The primary voltage is 240 V. Find the secondary voltage. [3]3 marks

  2. Assume an ideal step-down transformer that reduces 230 V to 12 V and supplies a 24 W lamp. Find the secondary and primary currents. [4]4 marks

  3. Power of 2.0 MW is sent through cables of resistance 5.0 Ω at 20 kV. Find the power lost in the cables. [4]4 marks

  4. State three ways to increase the induced e.m.f. when a magnet is pushed into a coil. [3]3 marks

  5. A current flows upward in a wire in a field pointing from left to right. Use Fleming's left-hand rule to state the force direction, where the page is viewed with the field in the plane of the page. [2]2 marks

  6. A coil has its magnet pushed in slowly and then pushed in four times as fast. State how the induced e.m.f. changes and why. [2]2 marks

  7. Two long parallel wires carry currents in the same direction. State whether the wires attract or repel and name the rule used. [2]2 marks

Key terms

TermMeaning
Electromagnetic inductionThe creation of an e.m.f. when a conductor cuts magnetic field lines or the field changes.
Lenz's lawThe induced current opposes the change that produces it.
SolenoidA long coil of wire that produces a field like a bar magnet.
CommutatorA split ring that reverses the current in a motor coil every half turn.
Slip ringA continuous ring that connects a rotating coil to the external circuit in an a.c. generator.
TransformerA device with two coils on an iron core that changes alternating voltage.
Step-up transformerA transformer with more turns on the secondary coil, giving a higher voltage.
Motor effectThe force on a current-carrying conductor in a magnetic field.

Avoid these mistakes

  1. Using a transformer with d.c. — it needs a changing current to induce a voltage.
  2. Confusing left-hand and right-hand rules — Fleming's left-hand rule is for motors.
  3. Saying a transformer creates energy — a step-up transformer lowers the current, so power does not increase.
  4. Forgetting slip rings give a.c. and a split-ring commutator gives d.c. output direction.
  5. Saying a stationary magnet in a coil induces a current — there must be relative motion or a changing field.

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